Measuring probes are used to determine various chemical and/or physical parameters of a medium such as for example the concentration of different substances, the pH value or the turbidity of a medium. Measuring probes of this kind often have an elongated body made for example of glass or a plastic material with a sensor element located at the end. To protect a measuring probe from getting damaged, measuring probes operating in a technical plant are usually arranged in a measuring probe housing. The known state of the art includes static probe housings as well as retractable probe housings with or without a treatment chamber.
Among others, measuring probe housings with immersion tubes are known which have a perforated protector basket in which the sensor element of the measuring probe is located during operation. The protector basket ends in a plug that faces towards the medium. The plug represents an important safety feature, because it allows a container that holds the medium to be sealed off against the ambient environment even when the immersion tube is retracted from the medium, and in the case of a retractable housing with treatment chamber, the plug allows the sensor element to be cleaned, calibrated and/or conditioned without the problem that treatment medium could escape from the measuring probe housing into the process medium, or that the process medium could enter into the measuring probe housing.
Depending on the field of application, different characteristics are required of a measuring probe housing or of the immersion tube to be installed in the housing. In particular for applications in the pharmaceutical industry, in the food industry and/or in biotechnology, a hygienic measuring probe housing would be desirable which would by design prevent germs and/or bacteria as well as residues of the process medium from attaching themselves to the inside or the surface, but would at the same time be safe and easy to handle.
It has been found that even when sterilizable materials are used such as stainless steel, or certain polymers such as Teflon, it is still possible that due to the design configuration, germs, bacteria and/or residues of process medium can attach themselves to or accumulate on the immersion tube and in particular on the protector basket, on the seals between the protector basket and the measuring probe and/or the seats for the seals. The accumulation or the attachment is helped by the formation of pockets and/or gaps which occurs on or inside the immersion tube already in the course of the technically demanding manufacturing process.
A special form-fitting seal is disclosed in published US application 2003/0019308 A1, by Oppermann (“Oppermann '308”), for an immersion tube without a safety plug and with a protector basket that is open at the end facing towards the medium. After the measuring probe has been inserted, this form-fitting seal forms a pressure fit in the opening and seals the interior space of the measuring probe housing without leaving a gap against the process medium. While the immersion tube according to this disclosure ensures a gap-free seal between the process space or process medium and the interior space of the measuring probe housing, it suffers from the drawback that in taking the measuring probe out of the housing, it is possible for process medium to escape from the process through the housing, or it is also possible for the sensor element to be damaged or destroyed during the process by particles or contaminations which can enter the protector basket through the terminal opening.
Oppermann '308 discloses a special type of molded seal for an immersion tube without a safety plug and with an protector basket, which is open at the end of the immersion tube facing a medium. This molded seal forms a press fit in the opening after insertion of the measuring probe and provides a gapless seal between the interior of the measuring probe housing and the process. The disclosed immersion tube does provide a gapless between the measuring probe housing and the process, but when the measuring probe is extracted from the measuring probe housing medium could leak from the process through the measuring probe housing or the sensor element could be damaged or destroyed by particles or contaminations, which can pass through the terminal opening into the protector basket.
It is therefore an object to develop a safe and hygienic immersion tube, as well as a measuring probe housing in which an immersion tube is arranged.